1. Field of the Invention
The present invention relates to a power amplifier, and more particularly to an apparatus and method for improving the efficiency of a power amplifier operating on the basis of a signal with a large peak-to-average power ratio (PAR).
2. Description of the Related Art
In conventional wireless communication systems such as cellular systems, a base station uses a power amplifier to communicate with user terminals distributed within a predetermined service area. The power amplifier amplifies a radio frequency (RF) signal for use in a communication system to transmit multiple signals and then outputs the amplified RF signal.
In particular, a cellular system using code division multiple access (CDMA) or orthogonal frequency division multiplexing (OFDM) technologies must transmit multiple modulated signals with a large peak-to-average power ratio (PAR) to multiple users connected thereto sharing the same frequency band. Because an existing power amplifier used to amplify an RF signal in this communication system must amplify all signals with large PAR and output the amplified signals, a significant amount of direct current (DC) power is consumed, resulting in an inefficient power amplifier, and increased manufacturing cost.
In order for the power amplifier operating under a large PAR signal to reduce average power loss, a peak reduction technique capable of typically reducing a signal peak value is used. This technique is described in “Clipping Noise Mitigation for OFDM by Decision-Aided Reconstruction”, by Kim et al., IEEE Communications Letters, Vol. 3, No. 1, January 1999 and in “Design Considerations for Multicarrier CDMA Base Station Power Amplifiers”, by J. S. Kenney et al., Microwave Journal, Vol. 42, No. 2, February 1999, both of which are incorporated herein by reference.
FIG. 1 is a block diagram illustrating an apparatus for improving the efficiency of a power amplifier using a conventional peak reduction technique.
The conventional peak reduction technique will now be described with reference to FIG. 1. When a signal with a large peak-to-average power ratio (PAR) is inputted from a baseband source signal generator (BSG) 110 to a peak reduced signal generator (PSG) 120, the PSG 120 detects a peak signal with an envelope value of a predetermined level or more from the inputted signal, reduces the level of the detected peak signal, and outputs a signal whose PAR is reduced. A peak reduced signal outputted from the PSG 120 is combined with a local oscillation signal from an RF local oscillator (RFLO) 135 by a quadrature modulator (QM) 130, such that a quadrature modulated RF signal is produced. An RF power amplifier (RFPA) 140 amplifies an output of the QM 130 and transfers the amplified output to an antenna.
The life span of a power amplifier can be improved as the PAR of a signal is reduced via the peak reduction technique. However, the peak reduction technique has problems in that a signal error rate and spectral regrowth become gradually higher as the signal's PAR is reduced, such that signal characteristics can be deteriorated thereby decreasing transmission efficiency. In other words, when the peak reduction technique is used, the improvement of the power amplifier efficiency is limited. Thus, a need exists for technology capable of effectively improving the efficiency of a power amplifier without increasing the signal error rate.